An enhanced image of telltale signs of pressure related stress on the San Bruno line made at 3,700 times magnification with a scanning electron microscope. Note white areas are ridges or striations, each marking steady but minute progress in the growth of a crack through the pipe's weld.

Photo: Ntsb

An enhanced image of telltale signs of pressure related stress on...

Image 2 of 4

Federal investigators examine a 40-foot section of pipeline on Glenview Drive in San Bruno, in the area where eight people were killed and 38 homes destroyed.

Photo: Paul Chinn, The San Francisco Chronicle

Federal investigators examine a 40-foot section of pipeline on...

Image 3 of 4

An enhanced image of telltale signs of pressure related stress on the San Bruno line made at 3,700 times magnification with a scanning electron microscope. Note white areas are ridges or striations, each marking steady but minute progress in the growth of a crack through the pipe's weld.

Photo: Ntsb

An enhanced image of telltale signs of pressure related stress on...

Image 4 of 4

An enhanced image of telltale signs of pressure related stress on the San Bruno line made at 3,700 times magnification with a scanning electron microscope. Note white areas are ridges or striations, each marking steady but minute progress in the growth of a crack through the pipe's weld.

Federal investigators' findings in the San Bruno pipeline explosion probe suggest that thousands of miles of long-buried and untested natural gas pipelines across the United States are at far greater risk of failure than the industry and government regulators have long maintained, experts say.

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In a detailed report released Jan. 21, the safety board identified a "progressive" crack on a seam running several feet along the San Bruno line as the point where the 30-inch transmission pipe ruptured, causing a fireball that killed eight people and destroyed 38 homes. The crack began at a shoddy weld that extended only halfway through the pipe wall, investigators said.

Although the report did not identify what forces were at work on that crack, Eiber and several other experts interviewed by The Chronicle said that only gas pressure - not outside forces such as earth movement - could account for the way the line failed and for microscopic, telltale markings seen on photographs of the weld.

Each one of those dozen or so marks - ridges and grooves that resemble a zipper - represents an instance in which the weld crack was forced open just a bit wider before it finally gave way.

Two of the marks were most likely caused by instances in 2003 and 2008 in which PG&E intentionally increased gas pressure on the line to what the utility believed was the most the pipe would safely hold, the experts said.

The other marks, however, are probably the result of years of routine pressure shifts - known as low-pressure cycling - that helped the crack to spread, they said.

Industry confident

The gas industry has long asserted that such pressure changes within legally established ranges do not pose a problem for the thousands of miles - perhaps more than 25,000 - of older gas transmission pipelines across the United States that, like the San Bruno line, have never undergone pressure safety testing.

Pressure limits on such lines have safety margins that account for dangers such as bad welds, the industry says. Federal regulations allowing such lines to go untested are predicated on the assumption that as long as the pipes are run below a certain level, weak welds are not at risk of failing.

Independent pipeline experts agreed that the latest federal findings about the San Bruno disaster cast doubt on that assumption.

"Unfortunately, PG&E was the first company to expose a problem like this on a major scale," said Richard Golomb, a mechanical engineer in San Francisco who spent four decades selling pressure-control equipment to PG&E and other pipeline companies. "I believe we are only seeing the tip of the iceberg."

Worrisome find

The implications for the natural-gas transmission industry are potentially enormous. Any change in federal pipeline inspection rules could require companies to conduct pressure testing that can cost as much as $500,000 per mile, for thousands of miles of pipe.

The cost of fixing the problem "is going to be a huge sum," Golomb said.

The ominous finding in the metallurgical analysis of the San Bruno pipeline was that a flawed weld put in the ground in 1956 could go undetected and gradually worsen - yet survive for decades before failing and causing a catastrophe.

If it can happen here, experts said, it can happen anywhere.

"PG&E is not the only natural gas company that has this problem," Golomb said. Every pipeline company in the United States and the world, he said, "could have these same problems."

Several experts who reviewed the government's metallurgical report on the San Bruno disaster said there were unmistakable signs that small, ordinary pressure changes over time drove the deadly crack down the pipeline's seam.

Investigators say the pipe's pressure was typically kept below 375 pounds per square inch, and PG&E says it never ran the line above its legal maximum set of 400 pounds per square inch. The utility has said it intentionally increased pressure twice to that maximum level, in 2003 and 2008.

Those pressure surges - which the utility defended as "very safe" and necessary to preserve "operational flexibility" - are being investigated as part of the federal probe into what caused the disaster, a spokesman for the National Transportation Safety Board said.

But Bea and other independent experts who examined photographs in the board's report say they can identify at least 10 other, smaller markings on the weld - each a signature of tiny progress in the depth of the crack on the line.

The welding is "so defective that even what we think are safe operating pressures had been acting to propagate the cracks," Bea said. "That is what is so scary."

PG&E's flawed records

The possibility exists that the vulnerability of the San Bruno line was an isolated case, caused by PG&E's ignorance of the true characteristics of its pipe.

The National Transportation Safety Board expressed alarm this month that PG&E had wrongly described the pipe as seamless. The fact that the San Bruno section had a seam, and was pieced together in several welded sections, means the line's true safety level could have been well under 400 pounds per square inch, the agency suggested.

The safety board's chairwoman, Deborah Hersman, said Wednesday that "decisions regarding inspections, operating pressures and risk management plans (for the San Bruno pipe) were all based on facts that were just plain wrong."

However, federal regulators also said other pipeline companies could have flawed documentation, and urged operators across the country to produce records for their lines and prove their maximum safety levels were justified.

In California, the state Public Utilities Commission ordered utilities to do so by March 15. Rep. Jackie Speier, D-Hillsborough, said PG&E President Chris Johns had told her the utility has been unable to find documentation for 30 percent of its gas transmission system in and around urban areas.

System's integrity

Eiber said the flawed weld on the San Bruno line and its ultimate failure calls into question PG&E's entire system.

"I cannot imagine anyone putting that piece of pipe in the ground," Eiber said. "This is a huge flaw. If they are this sloppy, I'd put a big question mark on the whole system."

PG&E has said it has operated its lines with a large margin of safety, but did not respond last week to questions based on the metallurgical report.

If federal regulators agree that only pressure changes were at work on the weld in San Bruno, that could trigger a "monumental" shift in gas pipeline regulation, said David Howitt, a UC Davis chemical engineering professor who studies pipeline failures.

Howitt said the San Bruno finding suggests that old lines and their flawed welds may be able to hold up for only so long before they start to give way.

'The fatigue cycle'

"If it is going to start happening, it is going to start happening now or in the next decade, because you are in the fatigue cycle," Howitt said.

"Now it is a waiting game," he said. "It is kind of like counting coffins. You are waiting to see if it happens again."

Howitt said it is still too early to determine the dynamics of the pressure changes in the San Bruno rupture. PG&E records on pressure increases - if the company kept them - will help investigators know just what amount of pressure left its microscopic signature behind.

Rules must change

Bea said the regulatory rules need to be changed, "or else we'll just expect more repetitions of this experience."

Federal regulators opted not to force pipeline companies to search for defective welds in the late 1980s after the National Transportation Safety Board issued warnings about a spate of weld failures. The board pushed for an inspection requirement, but regulators issued a voluntary advisory instead.

However, the federal government made it clear back then that pipeline companies should know the location of old welds.

"Most of us operators know where that pipe is, and we keep a closer eye on it," said Charles Yarbrough, vice president of rates and regulatory affairs for Atmos Pipeline-Texas in Dallas.

Regulators' dilemma

The main question now for government regulators is whether to force pipeline companies to conduct more high-pressure water tests on their lines to prove they are free of weld defects such as those exposed in San Bruno.

Utilities avoid the tests because they are extremely expensive - $125,000 to $500,000 per mile of pipe, according to PG&E - and require that a line be taken out of service for at least 10 days.

Under a 1970 federal law, water-pressure testing is done on all new lines when they are installed. But the law allowed lines already in the ground then - including the one in San Bruno - to be assigned pressure levels based on their history and characteristics, and not on any testing.

PG&E says it hasn't pressure-tested about 20 percent of its pipeline, according to initial estimates submitted to the California Public Utilities Commission. That would translate into more than 1,000 miles of pipe in Northern and Central California.

27,000 miles in U.S.

The Interstate Natural Gas Association of America, a trade organization, conducted a survey of its members indicating that about 27,000 miles of gas transmission lines nationwide have never been tested. The federal Pipeline and Hazardous Materials Safety Administration could not provide a total.

The industry insists such lines are safe.

Outside disturbances

Terry Boss, vice president of the interstate gas association, argued during a federal rule-making process in 2003 that any flaws in old welds could be designated as "stable" if there had been no pressure spikes or leaks on a pipe. The Pipeline and Hazardous Materials Safety Administration agreed and did not force companies to pressure-test their older lines.

Boss contended that weld problems would not be worsened by everyday pressure shifts within legal boundaries. He asserted that the most serious threat was from ground forces in an earthquake, landslide or contractor digging.

Boss said last week that his organization and others are still looking into potential outside disturbances that could have affected the San Bruno pipe. He said the markings found where the weld failed could have been the result of such disturbances rather than pressure changes.

A pressure surge doubtless caused the pipe to finally burst, Boss said - investigators say a mechanical failure boosted pressure on the San Bruno line from the normal 375 pounds per square inch to 386 pounds just before the blast - but that whatever weakened the pipe came from the outside. Possibly, he said, the problem was earth movement.

"In the end pressure does it, but (the pipe) has to be in such a weakened state because of other events," Boss said. He said his group's studies have consistently found that low-pressure cycling "doesn't show to be a problem with natural gas pipelines."

But Eiber, the pipeline integrity expert, is sure that earth movement was not to blame for the San Bruno disaster.

"Earth movement is not going to produce the (forces) that would trigger that crack to occur" on a seam weld, he said.

Richard Kuprewicz, a pipeline safety consultant in Redmond, Wash., said the San Bruno disaster is likely to change the regulatory landscape.

'A great concern'

"The assumption that gas transmission lines don't pressure cycle, I think it is going to look pretty ridiculous," Kuprewicz said.

Eiber said the lesson of San Bruno is that regulations assuming that weld risk can be discounted over time are flawed.

"It is difficult to anticipate what is out there," Eiber said. "It's a great concern. I think it's going to be a great concern to everybody."